Testing the waters: securing the UK’s undersea cables against grey-zone threats
Key Point Summary
Undersea cables underpin economic security and global prosperity in the digital age, carrying 99% of intercontinental data traffic. Undersea cables are vital for both civilian and defence infrastructure, including future AI-powered technologies.
A series of suspicious breakages in the Baltic Sea and Taiwan Strait indicate that China and Russia may be using undersea sabotage as part of broader grey-zone operations against their adversaries – including NATO and its member states.
This paper examines 12 suspected undersea cable sabotage cases from January 2021 to April 2025. Of the 10 with identified vessels, 8 are linked to China or Russia by flag or ownership.
The involvement of Chinese vessels in cable breakages in Europe, and Russian vessels near Taiwan, suggests plausible China-Russia coordination amid deepening ties in both the Euro-Atlantic and Indo-Pacific.
As a key hub in Euro-Atlantic cable infrastructure, the UK is a likely target for future Russian and Chinese grey-zone operations – posing a new and complex challenge for its maritime defence and surveillance systems.
The UK must be clear-eyed and proactive in addressing grey-zone threats to undersea infrastructure. Recommendations include:
Enhancing monitoring and surveillance: The UK should use NATO mechanisms to regularly share best practice and intelligence on undersea cable threats, including Russia and China’s shadow fleets, and extend cooperation to experienced partners like Taiwan and Japan.
Strengthening mechanisms for accountability: International law on undersea cables is outdated and insufficient. The UK should work with partners to strengthen accountability powers through utilising Port State Controls and publishing vessel blacklists. It must also tighten domestic laws and establish protocols for rapid pursuit, interdiction, and detention of suspect vessels.
Improving redundancy, repair and resilience: The UK government should work with private operators to ensure guaranteed access to cable repair vessels capabilities during crises or national emergencies, as well as strategic stockpiling of cable repair parts.
Policy Recommendations
(1) Enhancing monitoring and surveillance
Data sharing and analysis: The UK should establish forums with partners for sharing data on the identity and activities of ships which may pose a risk to undersea cables, including those believed to be within Russia and China’s ‘shadow-fleets’. Data shared could include AIS data, ship registration data, ownership data, port call data and satellite imagery. In particular, the UK should establish data sharing mechanisms on a bilateral level with Taiwan, which has devoted significant resources to such analysis, as well as Canada, Japan, South Korea and other partners. The UK should also seek a role in EU proposals for an Integrated Surveillance Mechanism for Submarine cables as part of ongoing discussions on the UK–EU Security and Defence Partnership. Such forums should also integrate data from private sector sources, such as cable operators and the shipping industry.
Exchange of best practice: The UK should conduct regular engagements with partners to exchange best practice on undersea cable monitoring and surveillance. Discussions could focus on areas such as use of early warning systems and other technologies, data collection and analysis (including AI-driven behavioural analytics), inter-agency cooperation, war gaming and rapid response tactics. Such exchanges could take place through relevant diplomatic or representative offices, or with partners including Japan and Taiwan through the Global Cooperation and Training Framework. Regular exchanges could also help open effective channels for communication and coordination in future crises.
Joint exercises and patrols: The UK should continue to play a leading role in NATO and JEF exercises and drills focussing on the surveillance and protection of undersea infrastructure, as well as considering including these operations as part of ongoing discussions within the UK–EU Security and Defence Partnership. Future exercises could focus on practising skills such as interdicting or boarding vessels suspected of undersea cable damage. While joint exercises and patrols are unlikely to be frequent or extensive enough to make a major contribution to surveillance, they can still play an important deterrent effect by demonstrating capabilities and showing united international support. Future deployments of UK vessels to the Indo-Pacific could include joint exercises and patrols with undersea infrastructure as a focus.
(2) Strengthening mechanisms for accountability
Publish blacklists of offending vessels: The UK should publish regularly updated blacklists of vessels involved in incidents of undersea cable sabotage and suspected vessels evading investigators. For maximum impact, such blacklists should be published in coordination with likeminded partners such as the EU or G7. Such lists could simply ‘name and shame’ offending vessels, with likely impacts on the designated vessels’ commercial operations, or be further strengthened by sanctions and punitive measures against those listed. Sanctions could include preventing blacklisted vessels from entering ports, insurers or other service providers in that country, or asset freezes against the vessel owners and operators, as provided for under the UK’s Sanctions and Anti-Money Laundering Act.
Amend the Paris MoU to incorporate cable-related compliance criteria: The UK should lead efforts to amend the Paris Memorandum of Understanding on Port State Control – of which it and most European countries are parties – to integrate cable-related incidents into ship risk profiles. This should include establishing an agreed reporting mechanism where coastal states alert Paris MoU members of cable incidents tied to specific vessels, prompting targeted PSC inspections. Investigations could examine patterns of anchoring, trawling, or transiting in protected cable zones for evidence of reckless or malicious behaviour. Data on the number of cable incident related inspections, and subsequent compliance with such inspections, can then be integrated into existing Ship Risk Profiles and the Flag State Performance List published by the Paris MoU.
Establish Joint Protocols for Pursuit and Investigation: The UK should work through NATO or other partnerships to adopt protocols allowing for the rapid pursuit, interdiction and detention of vessels suspected of cable damage. This should include the legitimate use of non-lethal, limited force against non-cooperative or evasive vessels in a manner consistent with the UN Charter. Examples of such measures could include the use of non-consensual boarding, non-lethal disabling technologies (such as entanglement nets or propeller traps) and warning shots. Publicly codifying procedures will both serve as a deterrent and demonstrate that responses are proportionate and consistent.
Amend domestic legislation to extend jurisdiction: The UK should update its domestic legislation to allow for extension of penal jurisdiction to its Exclusive Economic Zone for cases in which there is credible suspicion of damage to the UK’s critical undersea infrastructure. This could be achieved through updating the Submarine Telegraph Act 1885 to reflect modern maritime zones, namely clarifying that its provisions apply not just to the UK’s territorial waters but also its EEZ. This could also be achieved through signalling that the UK would be willing to use the National Security Act 2023, which already criminalises sabotage against the UK’s infrastructure conducted on behalf of a foreign power and is already extraterritorial in nature, for cases of suspected sabotage against the UK’s undersea cables.
Increase diplomatic pressure on non-cooperative flag states: Governments can put diplomatic pressure on China, Russia and other countries that fail to meet their good-faith obligations under UNCLOS to take meaningful measures to investigate and hold perpetrators of undersea cable damage to account. Such pressure could take place in established UNCLOS forums or through bilateral channels. Such an approach could be particularly effective for smaller, so-called ‘flags of convenience’ states. While many of these states gain from operating a light-touch regulatory model, they may still have some diplomatic and economic incentives to maintain a reliable reputation. It is likely that many of these countries have not chosen to take part in Russia and China’s shadow-fleet or grey-zone operations, and with the right diplomatic pressure, could be persuaded to distance themselves from these activities through ensuring meaningful investigation and accountability.
(3) Improving redundancy, repair and resilience
Ensure UK access to cable repair vessels in crisis scenarios: The UK government should secure guaranteed access to cable repair vessels to ensure rapid response capability during crises or national emergencies. While the Royal Navy’s new Multi-Role Ocean Surveillance Ship, RFA Proteus, has some repair capability, its primary surveillance function could limit its availability for cable repair during emergencies. The UK should consider emulating the United States’ model, where the government pays SubCom $10 million annually for continuous access to two cable repair vessels through its Cable Security Fleet. Alternatively, the UK could explore a cooperative arrangement with European partners under the EU’s proposed Cable Vessels Reserve Fleet, which envisions a shared pool of multi-purpose ships for emergency cable deployment and repair.
Support strategic stockpiling of cable repair parts with industry: The UK should work with cable operators to ensure pre-positioned depots of critical components – such as repeaters, spare cable, and universal joints – at key maritime chokepoints such as the West Coast of Ireland or English Channel. This would reduce the mean time to repair in high-traffic or high-risk regions and improve resilience against both accidental damage and deliberate interference. The planned development of Subic Bay in the Philippines into a regional cable repair hub, supported by SubCom and reportedly backed by U.S. investment, provides a useful model.
Review and upgrade standards and regulations: The UK should conduct a comprehensive review of its technical standards and regulatory frameworks for the construction, protection, and maintenance of undersea cables. This review should work with industry to assess current requirements for cable armouring and burial, as well as the effectiveness of warning systems, breakage detection, and the integration of new threat detection technologies. It should also examine licensing processes, including provisions for repair fleet availability, contingency planning, and the stockpiling of critical parts in the event of disruption.